The mechanism and regulation of the initiation of mammalian protein synthesis is being investigated by utilizing artificial template or hemoglobin mRNA-directed protein biosynthesis. Components from the rabbit reticulocyte have been fractionated and highly purified. Reactions of either the separate components or combinations of components are being studied to elucidate the sequential steps involved in protein synthesis. We plan to continue purification and characterization of those factors which have not been fully evaluated. With the homogeneous factors available and those to be obtained, studies with exogenous mRNA are planned to explore fully the differences in factor requirements for protein synthesis initiation with natural mRNA as opposed to artificial templates. In addition, studies are planned to examine the differences between mRNA and mRNP particles as well as factors which might effect the ratio of alpha and beta globin chain synthesis.